Metallized plasma path source



Feb. 13, 1968 w, LUEHRMANN ET AL 3,369,218

METALLIZED PLASMA PATH SOURCE Filed Nov 21, 1966 A STORED ELECTRICAL 3 \ENERGY SOURCE TANK l7 SOLENOID V" VALVE CH ECK VALVE INVENTORS WILLIAM H. LUEHRMANN WILLIAM H. PARKER ATTORNEYS 3,369,218 METALLHZED PLASMA PATH SOURCE William H. Luehrmann, Dallas, and William H. Parker, Richardson, Tern, assignors to Teledyne Industries, Inc., Geotech Division, a corporation of California Filed Nov. 21, 1966, Ser. No. 595,695 7 Claims. (Cl. 340-12) ABSTRACT OF THE DISCLOSURE An underwater acoustical source for etliciently converting electrical energy discharged between spaced immersed electrodes from a storage device into an intense plasma discharge to form a large steam bubble which subsequently collapses to provide an acoustical disturbance having an improved low-frequency content. The described embodiment includes means for periodically pumping a conductive liquid through a hose and jetting the liquid from one electrode toward and against theother to form a metallized path of higher initial conductivity between them, and then discharginga very high peak current through the path to vaporize it.

This invention relates to improvements in apparatus for generating underwater acoustical impulses, and more particularly relates to improved apparatus for increasing the efliciency of conversion of electrical energy to acoustical energy while decreasing the fundamental frequency thereof, especially in the type of seismic survey equipment employing a high-energy electrical discharge through water to set up a plasma bubble therein.

The prior art has suggested a number of different basic approaches to the problem of providing underwater seismic impulses, including explosions, magnetostrictive disturbances, electrical discharge disturbances, etc. It is to the latter type of approach that the improvements of the present invention are directed. The electrical discharge usually is from a source of stored electrical energy which can be discharged suddenly between two electrode means immersed in the water, and usually towed behind a vessel. The current from the source is introduced at one of the electrodes, passes through and ionizes the water in a region between the electrodes, and is returned to the source through another electrode which is usually located relatively close to the first electrode. The source of stored energy is advantageously of the capacitive type which is triggered by suitable means to apply great energy across the electrodes. The passage through the water of this energy ionizes the water in said region and creates conductive ions whose character normally depends upon the available constituents in the water, for instance C1 A large steam bubble is formed by the heat generated at the plasma discharge in the region between the electrodes and this bubble is sustained beyond the interval of time that the discharge current is passing therethrough, depending upon the time required to dissipate the heat in the bubble region. The size of the bubble and its rate of change determine the content of the acoustical shock wave, and in turn this size depends upon the amount of electrical energy per unit time that can be passed through the interelectrode region. Moreover, the duration of the bubble is the major contributing factor in the determination of the fundamental frequency of the seismic shock wave. The lower its frequency the better the penetration into bottom formations. Therefore it is desirable to create as large a bubble of as long a time-duration as possible.

The principal obstacle in the way of creating large bubbles of relatively long duration resides in the electrical resistivity of the discharge path, even after plasma is esnited States Patent 3,369,218 Patented Feb. 13, 1968 tablished in the region between the electrodes. The total resistance of the discharge path is the sum of the resistance of the conductors leading to the electrodes plus the resistance of the path between the electrodes. The latter resistance is much greater than the former, which can be minimized by the proper selection of the conductors leading from the electrical energy source to the electrodes. However, resistance of the current path in salt water never decreases into the milliohm range, and, in practical electrode configurations where the electrodes are separated by about a foot, the resistance is of the order of one ohm or more.

The improvement according to the present novel tech- 'nique involves the introduction of a highly-conductive liquid containing metallic particles or a metallic salt across the region between the electrodes prior to and/or during each energy discharge, and further involves an improved electrode configuration designed to insure contact of the conductive liquid with the electrodes due to the manner in which the water and liquid flow across the electrodes. As a result of the energy discharge, for example using a 25,000 joule source in salt water, bubble times of 30 to 35 milliseconds duration are typical, and this duration is approximately twice as long as is obtained using priorart techniques without the addition of conductive ions to the plasma region. The present technique employs a repeating cycle wherein, after each discharge through the plasma region, a highly-conductive liquid is injected into the region between the electrodes just prior to and/or during the next discharge. For instance, the discharge of a new impulse may take place at intervals of four to six seconds, most of the time between discharges being used to collect reflection data via towed geophone arrays.

It is the principal object of the present invention to provide novel apparatus for introducing a highly-conduc tive material into the region between electrodes having substantial spacings for the purpose of reducing the initial resistance of the path in order to obtain greater-energy discharge and therefore larger bubble sizes. This conductive material vaporizes as the electrical energy is discharged through it, and thus aids the flow of much higher peak currents. Greater discharge current has the elfect of significantly increasing the efficiency of the acoustical source, i.e., the percent of electrical energy transformed into acoustical energy. Since substantial electrode spacings can be employed with the present technique, and since a longer bubble time also results, a much higher-pressure acoustical impulse containing a lower fundamental frequency is generated in the water. This greater energy is particularly useful for the purpose of obtaining seismic reflection data from geological beds located, for instance, several miles below the water bottom.

It is another object of the invention to provide efiicient apparatus for increasing path conductivity to facilitate the establishment of an initial discharge of current between electrodes having substantial mutual spacings. Without such increased conductivity, a discharge between widely spaced electrodes will probably not attain adequate ionization therebetween. Moreover, larger spacing contributes to greater electrode life, since closely-spaced electrodes are eroded away more quickly than widerspaced electrodes passing the same electrical energy.

Still another object of this invention is to provide a system for creating an acoustical disturbance by discharging a large store of electrical energy between spaced electrodes in a virtually non-conductive fluid medium. The technique of injecting a conductive path between electrodes makes it practical to use the present acoustical source in fresh water for example, whereas it has formerly been considered useful only in a salt water environment.

a The size of the bubble whose collapse creates the acoustical shock wave depends upon the power of the electrical energy discharge creating it, and in fresh water there was simply not enough conductivity to establish a highpowered discharge.

Yet another object of the invention is to provide an efiicient working system wherein most of the conductiveliquid injecting equipment is located safely aboard the towing vessel so that it does not experience the destructive forces occurring every few seconds in the vicinity of the discharge region. Great attention has been directed toward providing an electrode array which is capable of withstanding these forces for a reasonable length of time and which is, to a limited extent, self protective. Even heavy brass electrodes last only a few hours before they must be replaced. Under these conditions, it is very advantageous to be able to place most of the operative mechanism in a safe place, substantially inboard of the towing vessel.

Other objects and advantages of the present improved method and apparatus will become apparent during the following discussion of the drawing, wherein:

FIG. 1 is a pictorial view showing a vessel engaged in towing apparatus for generating an underwater acoustic impulse by an electrical discharge through the water;

FIG. 2 is a diagram illustrating apparatus for feeding conductive liquid into the region between electrodes; and

FIG. 3 is a perspective view of an electrode structure including conductive liquid feeding means and a towing yoke.

Referring now to the drawing, FIG. 1 shows a vessel V moving across a body of water, the vessel carrying on board a source S of electrical energy connected by a cable C to the terminals A and B of two spaced electrodes 12 and 14. The vessel V can also carry on board seismic recording equipment connected by another cable to a suitable geophone streamer (the latter equipment not being illustrated in the present drawing). Alternatively, such a geophone array can be towed behind a different vessel, all of these techniques being well known in the prior art and forming no part of the present invention, which relates only to the generation of acoustical impulses in the water.

FIG. 2 shows the electrical source S connected through a suitable trigger T which operates to close, in effect, a switch to deliver the electrical energy from the source S directly across the spaced electrodes 12 and 14, respectively. For instance, assume that the charge is delivered as a unidirectional pulse to the electrode 12 and that the electrode 14 functions as a return path. It is also convenient to assume that the electrode 12 is towed through the water ahead of the electrode 14 so that the conductive liquid 18 which is fed from the electrode 12 will be carried in the direction of the electrode 14 by the forward motion provided by the vessel V.

In the illustrative embodiment shown in FIG. 2, the conductive liquid 18 is contained in a tank 19 which has an outlet connected to a motor driven pump 16 which may advantageously run continuously. The pump serves to deliver the conductive liquid under fairly high pressure to a tube 20 whose leading end is attached aboard the vessel, and whose trailing end is connected to a nozzle 12a in the leading electrode 12. In order to conserve the conductive liquid, a normally closed solenoid valve 17 is provided between the pump 16 and the tube 20, which valve is opened only when its winding 17a is energized by a cyclic timer and control unit K whose function is discussed below. A check valve 21 is provided in the tube 20 near the nozzle 12a to prevent the sea water from entering the tube 20. The spring (not shown) in the check valve also provides a small back pressure which, when the solenoid valve 17 is closed, prevents the conductive liquid in the tube from leaking out.

The tube 20 is bundled with wires 22 and 24 to form the cable C which tows the electrode assembly behind the vessel V, these wires comprising #1 gauge conductors carrying the impulses of electrical energy from the source S to the electrodes 12 and 14.

The conductive liquid 18 may comprise a solution of a conductive salt, such as copper sulphate, or it may comprise a suspension of metallic particles in an emulsion; or it may comprise a slurry, for instance of common salt, etc. It is also convenient to inject a conductive plastic stream such as a liquid solder across the region R.

Considerable difiiculty has been experienced in providing satisfactory electrodes 12 and 14 having the desired rigid spacing in a structure which could be towed stably, while at the same time providing electrode supports and structures which would not be eroded away at a prohibitive rate by the energy discharge, FIG. 3 shows a structure which has been found suitable and which includes two fiber glass bars 30 and 32 joined together by a pipe yoke 33 to which the fiber glass bars are attached by straps 34. The electrodes 12 and 14 straddle the fiber glass bars 32 and 30 to which they are respectively attached and thus tend to shield these supports from discharge erosion. Moreover the electrodes are provided with rounded surfaces facing toward each other, which surfaces do not tend to eat away unduly rapidly as a result of the electrical discharge.

The leading electrode 12 is connected to the hose 20 and has a bore and nozzle 12a extending through it and in communication with the bore of the hose 20. The liquid 18 passes as a jet out through the nozzle 12a and is carried toward the electrode 14 by the slip stream of water flowing around the electrode 12. The electrode 14 is streamlined, and its tear-drop shaped surface 14a tends to cause the water and the conductive liquid to hug against it according to well-known principles of laminar flow. The liquid jetting out of the nozzle 12a is directed in a way to improve its contact with the electrode 14.

During operation of the present apparatus, the cycle timer and control unit K opens the solenoid valve 17 for an interval sufficient to cause the liquid to feed from the nozzle 12a in the electrode 12 against and around the electrode 14. The valve 17 is soon closed again to stop the flow of liquid 18 from the pump 16, and an electrical discharge is then initiated by the timer and control unit K actuating the trigger T to close the switch 10. This entire cycle is repeated periodically as the vessel moves through the water in accordance with well-known seismic exploration techniques.

The present invention is not to be limited to the exact form shown in the drawing for obviously changes may be made therein within the scope of the following claims.

We claim:

1. Apparatus for generating underwater acoustical impulses, comprising:

(a) electrode means immersed in the water in mutually opposed spaced relationship to provide a region therebetween;

(b) a source of highly-conductive liquid;

(c) tubular means leading from said liquid source to said region;

(d) means for pumping the liquid through the tubular means, said tubular means being oriented with respect to said electrode means to cause the liquid to contact opposed electrode means and form a path of increased conductivity therebetween; and

(e) a source of electrical energy connected across said opposed electrode means, and having sufficient energy when actuated to vaporize conductive components of said liquid in said path.

2. In apparatus as set forth in claim 1, one of said electrode means having a metal bore in electrical contact therewith and directed toward an opposed electrode means; and said tubular means being connected to discharge a jet of said liquid through the bore toward said opposed electrode means.

3. In apparatus as set forth in claim 2, said opposed electrode means being streamlined in cross-sectional shape and being located in said jet such that the liquid passes around the latter electrode means in laminar flow which causes the liquid to hug and contact the streamlined electrode surfaces.

4. In apparatus as set forth in claim 1, said electrode means including a towing lyoke attached to support insulating bars spaced apart in the direction of towing and disposed normal thereto; and heavy metal electrodes attached to and straddlin each bar, the electrodes having streamlined outer surfaces, and the thicker portions of the streamlined electrodes facing toward each other.

5. In apparatus as setfforth in claim 4, the leading electrode in said towing yoke having a bore therethrough facing the next following electrode, and said tubular means being connected to discharge a jet of said liquid through the bore and-against said following electrode.

6. In apparatus as set forth in claim 1, timer and control means connected to intermittently actuate said energy source; valve means connected with said tubular means and controlling the flow of liquid therethrough; and said timer and control means being connected to control said valve means to permit flow of said liquid syn- UNITED STATES PATENTS 7/1951 Rieber 340-12 10/1961 Padberg 340-l2 RODNEY D. BENNETT, Primary Examiner. J. G. BAXTER, Assistant Examiner. 

